Modular push button switch assembly mounted on printed circuit board

ABSTRACT

A key switch consisting of a switch mounted on a circuit board. The switch has at least three contact elements mounted on a plunger which is normally biased away from the circuit board. When pressure is applied to a key mounted on the side of the plunger opposite the contacts, the contact elements are lowered into engagement with corresponding contact points on the circuit board permitting a coded output to be generated. If one of the contact points on the board is a common point and the contact element on the plunger adapted to engage the common point is recessed, a simple and effective means is provided for preventing spurious outputs from the switch.

United States Patent [191 Coppola MODULAR PUSH BUTTON SWITCH ASSEMBLY MOUNTED ON PRINTED CIRCUIT BOARD [75] Inventor:

[73] Assignee: Bunker Ramo Corporation, Oak

Brook, Ill.

[22] Filed: Nov. 1, 1971 [21] Appl. No.: 194,741

John Coppola, Trumbull, Conn.

[52] US. Cl 200/159 R, 200/168 B [51] Int. Cl. H0lh 3/12, I-I0lh 9/02 {58] Field of Search 340/365, 365 A; 197/102; 179/90 K; 317/101 C, 101 CC; 339/17 C;

200/1 R, 5 R, 5 A, 16 A, 159 R, 159 A, 159

B, 166 PC, 166 J, 168 C, 168 K, 168 B [56] References Cited UNITED STATES PATENTS 3,586,806 6/1971 Swisher 200/168 X 3,696,908 lO/l972 Gluck et a1. ZOO/DIG. 1 3,710,060 1/1973 Brevick. 200/166 PC 3,707,611 12/1972 Twyford... 200/5 R UX 3,749,872 7/1973 Foster. 200/159 A X 2,383,735 8/1945 Ray 317/101 C 2,568,535 9/1951 Ballard..... 339/17 C 2,619,215 11/1952 Guss 197/102 [451 May 7,1974

2,752,580 6/1956 Shewmaker 317/101 CC 2,885,602 5/1959 Emerson et al. 317/101 CC 3,696,408 10/1972 Bouchard et a1. 340/365 A 3,506,795 4/1970 Schmidt. 200/5 A 3,210,484 10/1965 Dorsey 179/90 K 3,641,286 2/1972 Berezowski.... 200/5 A 3,499,515 3/1970 Mikrut 197/98 3,240,885 3/1966 Grunfelder et a1. 200/5 A Primary Examiner-James R. Scott Attorney, Agent, or Firm-R. J. Kransdorf; D. R. Bair;

F. M. Arbuckle [5 7] ABSTRACT A key switch consisting of a switch mounted on a circuit .board. The switch has at least three contact elements mounted on a plunger which is normally biased away from the circuit board. When pressure is applied to a key mounted on the side of the plunger opposite the contacts, the contact elements are lowered into engagement with corresponding contact points on the circuit board permitting a coded output to be generated. If one of the contact points on the board is a common point and the contact element on the-plunger adapted to engage the common point is recessed, a simple and effective means is provided for preventing spurious outputs from the switch.

9 Claims, inrewial at i MODULAR PUSH BUTTON SWITCH ASSEMBLY MOUNTED ON PRINTED CIRCUIT BOARD This invention relates to a switch adapted for use with a circuit board and more particularly to a key switch or keyboard element including a switch and circuit board, the element generating a coded character output.

BACKGROUND OF THE INVENTION Key switches are used extensively in electronic keyboards of processor terminal devices and other related applications. Switches presently used in these applications have, however, suffered from a number of serious deficiencies. First, these switches, which normally include a reed switch and a magnet or other switch activating device, have proved to be the most trouble prone portion of the terminal device. The relatively high profile of these switches has led to alignment problems and this and other factors has contributed to a high incidence of switch sticking. Reliability has also been handicapped by the fact that the switch terminals, and frequently the switch mounting posts, have been soldered to the circuit board on which the switch is mounted. Poor solder joints have resulted in difficultto-detect intermittent problems. Stray solder from the soldering operation has necessitated expensive cleaning operations which, if not performed successful, have permitted short circuits to occur in the resulting device. The heat from the soldering operation has also led to board warpage and the soldering operation has introduced potential corrosion problems. Finally, the soldering of switches to the circuit board has made it difficult and expensive to remove and replace switches where a problem does develop.

Existing keyboard switches have also been relatively expensive both in terms of the parts required for the switch itself and in terms of the time and materials required for assembling the switches into a keyboard. These costs have normally run in excess of a dollar a key even in large quantitites. With most terminal keyboards having 60 to 70 keys, and some special keyboards having well in excess of 100 keys, it is apparent that the cost of the keys represents a significant portion of the total terminal cost. One factor in the high assembly cost of existing keyboards results from the binary nature of the switches utilized. The binary switch outputs must be encoded in a diode matrix or other encoder before being applied to a decoder. A keyboard element with inherent encoding capability could thus significantly reduce both the material and assembly cost of the device.

A limiting factor in reducing the cost of existing switches is the cost of the reed switch elements. However, if the sealed reed switches are eliminated, the resulting unsealed contacts are subject to contamination problems. A built in wiping action of the contacts as they make and break may overcome this problem.

Another problem, particularly with non-binary switches, is the generating of spurious outputs in response to improper or incomplete switch closure. Existing solutions to this switch teasing problem have been relatively complex and have thus further increased the switch cost. A simpler and less expensive solution to the switch teasing problem is thus required.

It is therefore a primary object of this invention to provide an improved circuit board switch suitable for use in keyboard applications.

A more specific object of this invention is to provide a switch of the type indicated above which has significantly higher potential reliability than existing switches.

A still more specific object of this invention is to provide a switch of the type indicated above which does not require solder joints to physically or electrically connect the switch to a circuit board.

Another object of this invention is to provide a switch of the type indicated above which is easier and signifi cantly less expensive to manufacture, assemble into a keyboard, and maintain in the field.

A further object of this invention is to provide a keyboard switch with a built-in encoding capability, eliminating the need of a separate encoding circuit.

Still another object of this invention is to providea keyboard switch with a low profile and to thus significantly reduce switch alignment problems.

A still further object of this invention is to provide a switch of the type indicated above which provides, in a simple and inexpensive manner, relative immunity to spurious outputs resulting from improper or incomplete (i.e., teasing) closure of the switch.

A feature of this invention is the provisions of an inherent wiping action in the switch contacts thereby eliminating potential contamination problems in an unsealed switch.

SUMMARY OF THE INVENTION In accordance with these objects this invention provides a switch adapted for use with a circuit board to form a keyboard element. The circuit board has a plurality of contact points at least one of which is common. The switch is removably mounted on the circuit board by use of a suitable resilient means. The major element of the switch is a plunger having at least three resilient contact elements on the side thereof adjacent the circuit board. The plunger contact elements are aligned to make physical and electrical contact with corresponding contact points on the board when the plunger is depressed. The plunger is, however, normally biased by a suitable means away from the board. Each contact element on the plunger is connected electrically to a contact element adapted to engage a common contact point of the board and the common contact element on the plunger is recessed slightly from the remaining contact elements. This later feature assures that all contacts are engaged before an output is generated, thus preventing spurious outputs. The contact points on the board are coded so as to provide an encoded output representing the character of the keyboard element.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a partially cutaway, partially exploded, front view of a keyboard element of the invention.

FIG. 2 is a top view of the keyboard element shown in FIG. 1.

FIG. 4 is a partial, cutaway, side view of a slightly modified version of the element shown in FIG. 1 with the switch in a partially closed position.

FIG. Sis a partial, cutaway, side view of the element shown in FIG. 4 with the switch in a fully closed position.

FIG. 6 is a perspective view of a switch contact assembly suitable for use with the elements shown in FIGS. 1 5. a

FIG. 7 is an exemplary semi-schematic top view of a circuit board and related electrical circuitry suitable for use with a preferred embodiment of the invention.

FIG. 8 is a view similar to FIG. 2, showing a form of the invention'in which proper plunger orientation is assured.

DETAILED DESCRIPTION vor tracks 24 formed on opposite sides thereof.

A piston 26 is mounted for vertical movement in housing 14 and consists of an upper portion 28 positioned in bore 18 and a lower portion of enlarged diameter 30 positioned in counterbore 22. Plunger 26 has a pair of flanges 32 which ride in tracks 24 to prevent rotation of plunger 26 in housing 14. Point contact of plunger 26 with housing 14 is provided by a pair of dimples 34 formed on the upper surface of each of the flanges 32. Plunger 26 has an upper counterbore 36 and a lower counterbore 38. A key-head supporting insert 40 is pressure fitted in bore 36. Insert 40 has a discshaped flange 42 and a key-head supporting stem 44. A key-head 46 is pressure fitted on stem 44. It should be noted that while in FIG. 1 Stem 44 is shown as a narrow rectangular-shape post, stem 44 may be hexangular in shape as shown in FIG. 4, may be in the shape of a cross, or may have any other desired shape to accommodate different types of standard key-heads to be fitted thereon. Plunger 26 is normally maintained in the position shown in FIGS. 1 and 3 by a spring 48 which is fitted over portion 16 of body l4 and has flange 42 resting thereon.

A contact assembly 50 is staked or otherwise secured to the under side of plunger 26. Holes 51 (FIG. 6) are provided in assembly 50 for this purpose. For the embodiment of the invention shown in the figures, contact assembly 50 consists of three contact fingers. As may be best seen in FIGS. 3 and 6, the two outer ones of these fingers 52 are facing in one direction while the center finger 54 is facing in the opposite direction. As may be bestseen in FIGS. 1 and 3, center contact 54 is slightly recessed from contacts 52. The reason for this recessing of contact 54 will be described shortly. Also from FIGS. 1, 3, and 6, it can be seen that each of the contact fingers 52 and 54 has a bifurcated end with a projection or radius being formed in each end fork to assure good electrical contact. Dimples 56 formed on the underside of each flange 32 limit the movement of plunger 26 toward board 12. The limited point contact of the plunger with the board reduces the possibility of the key sticking.

The final element on each of the switches 10 is a pair of mounting posts 60, each of which has a push nut fastener or toggle spring 62 mounted thereon. Springs 62 are adapted to becompressed as posts are passed through openings 64 in board 12 and to expand as shown in FIG. 3 when the switch is mounted on the board to hold the switch in place. Since the upper half 66 of each of the springs 62 is also angled, upward pressure appliedto a mounted switch is effective to cause springs 62 to again be compressed, permitting a switch 10 to be removed. from board 12.

In addition to a pair of holes 64, the portion of board 12 associated with each keyboard element also includes a pair of outer printed circuitcontacts 68 (FIGS.

1 and 7), a center or common printed circuit contact 70, a printed circuit spacing dot72 adjacent to each hole 64, and a pair of printed circuit spacing rings 74 at the two corners of the switch which do not contain holes 64.'Since a switch 10 maybe positioned over printed circuit wiring on board 12, in the absence of spacing dots and rings 72 and 74, the switch might not be resting securely on the board and might have a tendency to rock. Printed circuit material 72 and 74 is of the same depth as the printed circuit wiring and assures that rocking of the switch does not occur. The material for spacers 72 and, 74 may be either conductive or nonconductive.

Referring now particularly to FIG. 7, the wiring required on board 12 to encode four exemplary characters (A, B, Q, and R) in American Standard Code for Information Interchange (ASCII) is shown. In ASCII code, each character may be fully characterized by his row and column position in a matrix. Character A is in row 1 column 4 and character B in row 2 column 4. Characters Q and R are in rows 1 and 2 of column 5. It is therefore seen that the left hand contact points 68 for the A and B characters are tied together with printed circuit wiring 78. This line is schematically shown connected as the column 4 (C4) input to character coder 80. Similarly, the left-hand contact points 68 for the Q and R characters are tied together with a printed circuit line 82, this line being schematically shown as the column 5 (C5) input to coder 80. Since characters A and Q are both in row 1 of the ASCII matrix, the right hand contact points 68 for the characters A and Q are tied together by a common line 84 which is schematically shown as the row 1 (R1) input to coder 80. Similarly, the right hand contact points 68 for the B and R characters are connected together by a printed circuit line 86 whichis shown schematically connected as the row 2 (R2) input to coder 80. The common center contacts 70 for all four characters are shown tied together by a printed circuit line 88 which is schematipression into the seven or eight bit ASCII code for the character.

While in FIG. 7, printed circuit lines such as 82 and 84 are schematically shown as crossing, such crossings would not occur on an actual board. Various standard techniques, such as for example, doing some wiring on the opposite side of the board with plated through interconnecting holes, could be utilized for this purpose.

To assemble a keyboard element of the type shown in the figures, the posts 60 of the switch are inserted in corresponding holes 64 of board 12, and the switch is pushed down until the elbow of toggle spring 62 passes the bottom of the board and expands to lock the switch in place. It is apparent that the switch must be inserted with the proper orientation so that contacts 52 are over contacts 68 and contact 54 over contact 70. This may be accomplished by providing a key on one of the posts 60 or springs 62 with a mating groove in a hole 64. However, for the preferred embodiment of the invention shown in the figures, proper orientation is obtained by making one post (post 60A) larger then the other, i

with the mating holes 64 likewise being of different size. Orientationarrows 90 are provided on the top of housing 14 to assist in making the proper orientation decision when assembling the element.

In assembling the switch it is also important that the plunger be properly oriented in housing 14 for the same reason indicated above. As shown in FIG. 8, the flanges 32 and tracks 24 may also be of different size so as to assure proper plunger orientation.

In operation, the switch is initially in the position shown in FIGS. 1 and 3 being biased to that position by the action of the spirng 48. When pressure is applied to a key top 46, plunger 26 is depressed to the position shown in FIG. 4. In this position, contacts 52 on plunger 26 are in contact with contacts 68 on board 12, but contact 54 is not yet touching contact 70. Since contact 70 is the common contact, an open circuit condition still exists at this time. Contact 54 being recessed from contacts 58 thus prevents spurious outputs from being generated as a result of the partial depression of plunger 26.

Further depression of plunger 26 causes fingers 52 to be compressed and brings finger 54 into contact with contact 70. The geometry of fingers 52 and 54 is such that, as they are compressed, they move across contact 68 and 70. This wiping action removes accumulated dirt, film, and other contaminants from the contacts, assuring reliable operation of the switch. FIG. 5 shows the switch in its fully closed condition.

When pressure is removed from key 46, spring 48 returns the switch to the position shown in FIG. 3. During switch return, the'sequence of operations is the reverse of that described above during key closure. There is thus a second wiping of the fingers on contact 68 and 70, this wiping occurring in the opposite direction from that for switch closure. Further, contact is broken by finger 54 before contact is broken by fingers 52, thus preventing a spurious code from momentarily being applied to coder 80. g

A switch and keyboard element has thus been provided which is capable of directly generating coded outputs, eliminating the need for a diode matrix or other character encoding circuit. Further, since the switch has a minimum of parts, not requiring either a reed switch or a magnet, the materials cost for the 6. switch are minimal. The cost of the switch 10 has been found to be roughly 30 percent of the cost of existing reed keyboard switches. Similarly, since there are no soldering operations involved in the assembly operation, and encoding circuitry may be eliminated, the cost of assembling the switch into a keyboard element has also been found to be roughly 20 to 30 percent of the cost for existing reed switches. The resulting keyboard element may thus be obtained for roughly 25 percent of the cost of existing keyboard elements.

Since space need not be provided for reeds, magnets and other elements, this switch may provide a relatively low profile, thereby significantly reducing switch alignment problems. The lower profile also reduces the likelihood of switch sticking and related problems. Further, since electrical contact is made through contact assembly 50 mounted directly on the plunger and mechanical securing of the switch is achieved through a toggle spring 62, soldering, with the numerous problems incident thereto mentioned above is totally eliminated. A switch which may be easily installed or removed in the field for maintenance or other purposes is thus provided. Other advantages which have been indicated above include an inherent wiping action which improves reliability by removing contaminants from the contacts and an anti-teasing action achievedby the recessing of center contact 54 to prevent spurious outputs from being generated. Since the anti-teasing mechanism introduces no additional parts into the switch, this desirable feature is achieved with minimal if any effect on switch cost. Insert 40 also permits the key tops.

While theinvention has been shown above with reference to a preferred embodiment thereof, it is apparent that various changes in the details of this structure are possible. For example, a dimple rather than a radius might be provided at the end of each contact finger 52, 54, and, for some applications, contacts having a different shape might be utilized. Similarly, means other than spring 48 might be provided for biasing the key to its open position, and a resilient means other than springs 62 might be utilized for securing the switch to the board. While the discussion above has been with reference to keyboards, it is apparent that the element of this invention might be utilized in any application where a key generating a coded output is required. It is also apparent that, while a single contact assembly 50 having a common contact and two additional contacts is shown for the preferred embodiment of the invention, additional contacts might be provided in the assembly 50 and additional assemblies 50 might be provided on a single switch plunger. The only limitation is that each contact finger 52' which is not adapted to make contact with a common contact on the board be electrically connected to a finger 54 which is to make contact with a common contact 70. The key switch element of this invention is thus adapted to prokey switch to be easily modified to accept a variety of -vide a wide variety of coded'outputs.

While the invention has been particularly shown and described above with reference to a preferred embodiment thereof, it is apparent that various modifications in the details of each of the elements shown may be made by those skilled in the art without departing from the spirit and scope of this invention.

What is claimed is: H V M rr W 1. A modular contact unit for cooperating with conductive contact pads on a circuit board to form a switch mechanism, said contact unit comprising a housing of electrically insulating material, externally substantially polygonal in plan shape, having a centrally located circular sleeve extending upwardly therefrom, a plunger extending through said sleeve from the inside of said housing, the inside of said housing having a bore of generally circular shape, open at the bottom, with a pair of axially extending grooves formed in the sides of said bore adjacent to diagonally opposite corners of the outside of said housing, said plunger having a lower portion of enlarged diameter freely movable axially in said bore, said lower portion having flanges fitting in said I grooves to prevent rotation of said lower portion in said bore, there being a shoulder between the inner end of said sleeve and said bore, spring means tending to move said plunger to a raised position, a multiple leaf spring contact carried on said lower portion, adapted for engagement with contact pads on a circuit board, and means for mounting said key switch to a circuit board having contact pads and holes appropriately located with reference to said pads, said mounting means consisting of a pair of posts extending from the bottom of said housing at the two corners thereof other than the corners adjacent to which said axially extending grooves are formed, and detent spring retainers carried by said posts, engageable with a circuit board, through said holes in said circuit board.

2. A modular contact unit as defined in claim 1, wherein said plunger has an axially extending opening in the upper end thereof, and carries a key-head supporting insert press fitted in said opening, said insert having a radially extending flange, and said spring means comprises a helical compression spring surrounding said sleeve portion, and bearing at its respective ends upon said flange and said housing.

3. A modular contact unit as defined in claim 1, wherein said detent spring retainers consist of split rings frictionally fitted on said posts, said rings each carrying a plurality of axially extending outwardly bowed leaf springs, the ends of said springs opposite said rings bearing against said posts adjacent to said housing.

4. A modular contact unit as defined in claim 1, wherein one of said posts is larger thanthe other, assuring a predetermined rotational orientation when said contact unit is mounted on a circuit board having correspondingly sized holes.

5. A modular contact unit for cooperating with conductive contact pads on a circuit board to form a switch mechanism, said contact unit comprising a housing of electrically insulating material having an axially extending bore therein, a spring-biased plunger slidably carried in said bore, a portion of said plunger extending externally of said housing and adapted for connection to a key head, a multiple finger spring contact carried on said plunger.within said bore, said contact having two fingers extending toward one side of said housing and at least one finger extending toward the opposite side of said housing, keying means on said housing and said'plunger controlling the rotational orientation of said plunger and said contact with respect to said housing, and means for mounting said contact unit on a circuit board having contact pads and receiving holes appropriately located with reference to said pads, said mounting means consisting of a plurality of posts extending from the bottom of said housing, en gageable in said receiving holes in said circuit board, and detent means retaining said posts in said holes, being receivable in said holes only in a predetermined orientation of said housing with respect to said board, and hence with a predetermined orientation of said contact leaves with respect to contact pads on the circuit board.

6. A modular contact unit as defined in claim 5, wherein said plunger has an axially extending cavity in the portion thereof extending externally of said housing, and carries a key-head supporting insert in said cavity, said insert having a radially extending flange, and a helical compression spring surrounds said plunger, and bears at its respective ends upon said flange and said housing.

7. A modular contact unit as defined in claim 5, wherein said keying means comprises interfitting elements on said plunger and said housing limiting assembly of said plunger into said housing to one position of rotational orientation.

8. A modular contact unit as defined in claim 5, wherein said keying means comprises radially extending flange elements on said plunger at diametrically opposite sides thereof, and axially extending tracks internal to said housing, receiving said flange elements, said flange elements and corresponding tracks being of different sizes on opposite sides of said plunger, whereby said plunger. is rotationally polarized with reference to ai bas e- 9. A modular contact unit as defined in claim 1, wherein said lower portion of enlarged diameter has projections extending upwardly therefrom and engageable with said shoulder when said plunger is moved upwardly by said spring means, said projections determining a minimum spacing of said portion of enlarged diameter from said shoulder. 

1. A modular contact unit for cooperating with conductive contact pads on a circuit board to form a switch mechanism, said contact unit comprising a housing of electrically insulating material, externally substantially polygonal in plan shape, having a centrally located circular sleeve extending upwardly therefrom, a plunger extending through said sleeve from the inside of said housing, the inside of said housing having a bore of generally circular shape, open at the bottom, with a pair of axially extending grooves formed in the sides of said bore adjacent to diagonally opposite corners of the outside of said housing, said plunger having a lower portion of enlarged diameter freely movable axially in said bore, said lower portion having flanges fitting in said grooves to prevent rotation of said lower portion in said bore, there being a shoulder between the inner end of said sleeve and said bore, spring means tending to move said plunger to a raised position, a multiple leaf spring contact carried on said lower portion, adapted for engagement with contact pads on a circuit board, and means for mounting said key switch to a circuit board having contact pads and holes appropriately located with reference to said pads, said mounting means consisting of a pair of posts extending from the bottom of said housing at the two corners thereof other than the corners adjacent to which said axially extending grooves are formed, and detent spring retainers carried by said posts, engageable with a circuit board, through said holes in said circuit board.
 2. A modular contact unit as defined in claim 1, wherein said plunger has an axially extending opening in the upper end thereof, and carries a key-head supporting insert press fitted in said opening, said insert having a radially extending flange, and said spring means comprises a helical compression spring surrounding said sleeve portion, and bearing at its respective ends upon said flange and said housing.
 3. A modular contact unit as defined in claim 1, wherein said detent spring retainers consist of split rinGs frictionally fitted on said posts, said rings each carrying a plurality of axially extending outwardly bowed leaf springs, the ends of said springs opposite said rings bearing against said posts adjacent to said housing.
 4. A modular contact unit as defined in claim 1, wherein one of said posts is larger than the other, assuring a predetermined rotational orientation when said contact unit is mounted on a circuit board having correspondingly sized holes.
 5. A modular contact unit for cooperating with conductive contact pads on a circuit board to form a switch mechanism, said contact unit comprising a housing of electrically insulating material having an axially extending bore therein, a spring-biased plunger slidably carried in said bore, a portion of said plunger extending externally of said housing and adapted for connection to a key head, a multiple finger spring contact carried on said plunger within said bore, said contact having two fingers extending toward one side of said housing and at least one finger extending toward the opposite side of said housing, keying means on said housing and said plunger controlling the rotational orientation of said plunger and said contact with respect to said housing, and means for mounting said contact unit on a circuit board having contact pads and receiving holes appropriately located with reference to said pads, said mounting means consisting of a plurality of posts extending from the bottom of said housing, engageable in said receiving holes in said circuit board, and detent means retaining said posts in said holes, being receivable in said holes only in a predetermined orientation of said housing with respect to said board, and hence with a predetermined orientation of said contact leaves with respect to contact pads on the circuit board.
 6. A modular contact unit as defined in claim 5, wherein said plunger has an axially extending cavity in the portion thereof extending externally of said housing, and carries a key-head supporting insert in said cavity, said insert having a radially extending flange, and a helical compression spring surrounds said plunger, and bears at its respective ends upon said flange and said housing.
 7. A modular contact unit as defined in claim 5, wherein said keying means comprises interfitting elements on said plunger and said housing limiting assembly of said plunger into said housing to one position of rotational orientation.
 8. A modular contact unit as defined in claim 5, wherein said keying means comprises radially extending flange elements on said plunger at diametrically opposite sides thereof, and axially extending tracks internal to said housing, receiving said flange elements, said flange elements and corresponding tracks being of different sizes on opposite sides of said plunger, whereby said plunger is rotationally polarized with reference to said housing.
 9. A modular contact unit as defined in claim 1, wherein said lower portion of enlarged diameter has projections extending upwardly therefrom and engageable with said shoulder when said plunger is moved upwardly by said spring means, said projections determining a minimum spacing of said portion of enlarged diameter from said shoulder. 